Tobias Friedrich and Axel Timmermann (Honolulu, HI, USA)
With 3 Figures
The mechanisms for the orbital-scale co-variability of atmospheric CO2, continental ice sheets and temperature during the Pleistocene still remain elusive. Numerous studies have demonstrated that the last glacial termination records of marine, terrestrial and atmospheric radiocarbon activity (∆14C) can provide valuable insights into past ocean-ventilation changes, the exchange of carbon between the ocean and the atmosphere and the origin of a hypothetical glacial ocean-carbon reservoir (Sarnthein et al. 2013, Broecker and Barker 2007, Oka-zaki et al. 2012). Such a glacial ocean-carbon reservoir – created by a sluggish ocean circu-lation and thus a poor venticircu-lation – should be identifiable as a large negative ∆14C anomaly in benthic foraminiferal CaCO3 records. Unfortunately, recent studies are inconclusive and contradictory with respect to the existence and/or a deglacial ventilation of such a reservoir.
Whereas some studies clearly document regional evidence for glacial 14C-depleted deep wa-ter (Galbraith et al. 2007, Skinner et al. 2010 and others), other studies do not find such evidence (Broecker et al. 2004, Lund et al. 2011, Okazaki et al. 2012 and others).
Probably, the most controversial features found in the deglacial radiocarbon records (Fig. 1) are given by large millennial-scale excursions in radiocarbon ages during the HE1-BA-YD transition seen at Baja California (Marchitto et al. 2007), in the eastern equatorial Pacific (Stott et al. 2009), the North Pacific (Rae et al. 2014) and the Nordic Seas (Thor-nalley et al. 2011). These anomalies have previously been interpreted in terms of a redis-tribution of 14C-depleted water in the context of the ventilation of a large-scale deep-ocean carbon reservoir. The overall lack of consistency in the available ∆14C records, however, sug-gests that local features such as regional ocean circulation changes may have contributed significantly to the temporal evolution of some ∆14C records.
To test this hypothesis in the context of the Nordic Sea records (Thornalley et al. 2011) we conduct model experiments (using the LOVECLIM Earth system model) that mimic the large-scale ocean circulation changes during Heinrich event 1. A substantial weakening of the AMOC is generated and lasts for about 3,300 years which is in good agreement with the estimated duration of HE1 (Bard et al. 2000, McManus et al. 2004). As a result of the overturning circulation slowdown in the North Atlantic ventilation of the deep Arctic ocean decreases significantly (Fig. 2).
The simulated deep-ocean ∆14C-anomalies in the Arctic ocean reach –250 –300‰ after about 2,500 years of the AMOC weakening. The increased residence time of the water is asso-ciated with a substantial increase in DIC and thus a surplus in carbon storage. Given that
simu-Tobias Friedrich and Axel Timmermann
66 Nova Acta Leopoldina NF 121, Nr. 408, 65 – 69 (2015)
Fig. 1 Selected ∆14C data from recent publications and respective core locations. The black line in the time-series panels indicates the INTCAL09 calibration curve (Reimer et al. 2009).
Fig. 2 (A) Simulated AMOC anomaly in Sv. (B) ∆14C-anomalies in ‰ (shaded) and DIC anomalies in µmol/kg (contours) horizontally averaged over Arctic ocean.
Effects of Sea-Ice and Ocean-Circulation Changes on Deglacial Deep-Ocean Radiocarbon Trends
Nova Acta Leopoldina NF 121, Nr. 408, 65 – 69 (2015) 67
lated water mass ages in the – sea-ice covered – Arctic ocean are in the order of ~1,800 –2,200 years (not shown) our preliminary results reveal that during a long-term AMOC weakening such as HE1, water mass ages in the deep Arctic ocean can reach 5,000 years and more.
We can now ask: What would happen to this extremely old water once the AMOC re-sumes?
Figure 2 reveals that ∆14C-anomalies in the Arctic ocean disappear almost immediately in the wake of the AMOC strengthening. This suggests that the old Arctic water is replaced and flushed out into the Nordic Seas. A flushing of this extremely old water in turn could po-tentially explain the large centennial-scale radiocarbon age variations documented by Thor-nalley et al. (2011). A more advanced version of the model experiment that is currently conducted has also been equipped with an artificial water mass tracer that will allow us to discern between water of Arctic and Southern Ocean origin at the Nordic Sea core locations.
Even though the consequences for the interpretation of ∆14C-records remain elusive at this point, our preliminary results already document some interesting conclusions:
– The Arctic ocean may have played an important role in shaping deglacial North Atlantic
∆14C-records but only a small role in contributing to the deglacial atmospheric CO2 in-crease.
– Large-scale ocean circulation changes can generate regional ventilation and thus
∆14C-anomalies of substantial magnitude that have absolutely nothing to do with the glob-al-scale glacial carbon reservoir.
– A regional sea-ice cover (such as simulated for the glacial Arctic) acts as a lid for air-sea gas exchange and can significantly contribute to the ageing of waters.
Figure 3 compares the simulated annual-mean Northern Hemispheric sea-ice extent for pres-ent-day and LGM conditions. Whereas under prespres-ent-day conditions the sea-ice margin in the Western North Atlantic is close to Denmark Strait, the simulated sea-ice extent was significantly larger during the LGM. A similar result is obtained for the Southern Hemisphere (not shown).
Fig. 3 Simulated sea-ice and snow cover as well as continental ice sheets for present-day (A) and LGM (B) condi-tions using a transient simulation of the last eight glacial cycles (Friedrich et al. 2015, in prep.).
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68 Nova Acta Leopoldina NF 121, Nr. 408, 65 – 69 (2015)
What is the effect of this sea-ice lid on ocean ventilation and marine radiocarbon? In order to determine the magnitude of this “lid-effect” we designed and conducted model experiments in which the sea-ice cover climatologies for present-day and LGM conditions respectively are prescribed in the air-sea gas exchange parametrization of the model. This set-up allows us to solely account for the “lid-effect” whereas other critical effects of sea-ice on albedo, brine rejection, wind stress transfer and surface heat fluxes remain completely unchanged. For ex-ample, the air-sea gas exchange in a present-day simulation is diagnosed using an LGM sea-ice cover climatology and the resulting change in ocean ventilation and marine radiocarbon is documented. The interesting and surprising results will be (partly) presented during the talk.
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Tobias Friedrich, Ph.D.
International Pacific Research Center
School of Ocean and Earth Science and Technology University of Hawaii
Honolulu, Hawaii 96822 USA
Phone: +1 808 9567385 Fax: +1 808 9569425 E-Mail: tobiasf@hawaii.edu Prof. Axel Timmermann, Ph.D.
International Pacific Research Center
School of Ocean and Earth Science and Technology University of Hawaii
Honolulu, Hawaii 96822 USA
Phone: +1 808 9562720 Fax: +1 808 9569425 E-Mail: axel@hawaii.edu
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